Studies on mating patterns of marine crustaceans are rare, but the few studies on brachyuran crabs to date suggest that polyandrous mating is uncommon. In the present study, we used 6 microsatellite loci to genotype 8–16 larvae obtained from each of 10 gravid females of the crab Ucides cordatus (Brachyura: Ocypodidae). Based on the number of non-maternal alleles detected among the offspring, at least 40% of the females were inseminated by more than one male. We suggest forced copulation and maintenance of sperm in spermathecae throughout the year as possible causes of polyandry in U. cordatus